Airbag systems in common usage typically have a fixed inflation rate followed by a deflation rate determined by bag porosity and vents in the bag as well as by the momentum of the occupant. The accompanying restraint forces are generally sufficient to cushion the crash impact for a large person during a severe crash. Such high restraint forces are unnecessary for less severe crashes or for smaller people, and are often undesirable for a small person particularly when out of the normal seating position. Ideally, the inflated airbag will gradually collapse in a manner to maintain an effective and uniform force throughout its deflation period, and that it will prevent contact of the occupant with the steering wheel or other vehicle structure.
It has been proposed, for example in the U.S. patent application Ser. No. 08/695,814, filed Aug. 5, 1996 by Liu entitled "Control Method for Variable Level Airbag Inflation" assigned to the assignee of this invention, to inflate the airbag in a selected number of stages according to crash severity. This does not take into account the mass of the occupant nor does it ensure that the restraint force is uniformly applied throughout the airbag deflation.